1. The Field of the Invention
The present invention relates to a method and apparatus for substantially eliminating unequal phase splitting of wet steam at piping junctions and, in particular, to a system which separates the liquid and vapor phases upstream of the junction and then recombines them at the inlet of a piping junction.
2. The Prior Art
There is a need for a simple method and apparatus to control phase splitting which occurs at piping junctions in wet steam distribution systems, particularly at impacting T-junctions. Such an apparatus, if simplified, would be particularly useful in controlling steam quality and thereby the amount of heat which is available since more heat is transmitted by the vapor phase of the steam than by the liquid phase.
Generally, as pressurized wet steam flow flows through a piping system, there is a tendency for the steam to separate into its vapor and liquid phases. The separation occurs with the heavier and slower liquid phase becoming annular and adhering to the piping walls while the lighter and faster gaseous phase moves axially through the piping. This results in steam of unequal quality coming off, for example, the arms of an impacting T-junction.
It is important, therefore, as a matter of economic practicality that a means be instituted in the steam pipeline to prevent unwanted phase separation and promote homogeneity of the steam, particularly where it comes into and out of piping junctions.
Phase splitting is a phenomena of two phase vapor (or gas) and liquid flow that occurs at all piping junctions such as impact T's, branch T's, Y's, crosses, manifolds, etc. In standard junctions the liquid and vapor phases do not diverge in equal mass proportions except in junctions with symmetrical flow, such as in impact T's where the vapor mass rates are equal in each junction outlet (a vapor extraction ratio of 0.5). This is important, for example, in steamflood distribution systems, used for enhanced oil recovery, where it is desirable to deliver nearly equal steam quality throughout the entire distribution system. Steam quality is a measure of the proportion of the total mass that is vapor. The vapor extraction ratio is defined below.
Vapor Extraction Ratio is, ##EQU1## where, F.sub.g3 =Vapor Extraction Ratio
M.sub.v1 =Inlet mass rate of the vapor phase PA1 M.sub.v2 =Outlet branch 2 mass rate of vapor phase PA1 M.sub.v3 =Outlet branch 3 mass rate of vapor phase
Numerous studies investigating phase splitting have been conducted and various devices to equalize or control phase splitting have been tried. However, only a few of these ideas have been implemented in the design of new steam distribution systems and none have become standard practice throughout the industry. Still fewer of these methods are commonly encountered as "fixes", to minimize or control phase splitting, in distribution systems which were built before phase splitting was widely understood. The method disclosed here meets the criteria required of a "fix" in that it requires no operator action, creates minimal pressure drop, and is both inexpensive and effective.
An example of where the present invention would be particularly useful is secondary recovery of hydrocarbons from marginal fields or heavy oil reserves that require a degree of stimulation to achieve satisfactory flow of crude petroleum. In such operations steam is sent through a patterned array of injection wells to heat the formation being treated and drive the hydrocarbons towards a production well. The steam quality will directly affect the formation heatup effect and thus the efficiency of the recovery operation. The vapor phase of the steam will have the most heat and therefore have the greatest effect on the formation. Thus it is desirable to have steam of uniform quality injected into all portions of the formation.